1. Field of the Invention
The present invention relates to improving the heat resistance of polymers comprised of vinyl chloride. Such polymers are typically subjected to elevated temperatures, both during the various operations for the mechanical shaping thereof and also while in use.
2. Description of the Prior Art
It is well known in the art that polymers and copolymers of vinyl chloride are utilized for the manufacture of various shaped articles by molding, extrusion and casting processes which require high temperatures in order to bring the polymer to a sufficiently soft state. At these temperatures, which can be as high as 180.degree.-200.degree. C., polymers based on vinyl chloride undergo considerable degradation which manifests itself in an adverse color change and in a diminution in their mechanical properties. In order to avoid these adverse changes or effects, it has been proposed to add to the polymer feed heat stabilizers such as, for example, metal chelates of dicarbonyl compounds which can be used individually or together with salts of carboxylic acids and such metals as calcium, lead and cadmium [compare U.S. Pat. Nos. 2,307,075 and 2,669,548]. Stabilizers comprising salts of heavy metals and higher fatty acids, in conjunction with certain organic phosphites [U.S. Pat. No. 2,564,646] or with certain polyols [U.S. Pat. No. 2,711,401] have also been used. However, these various means are not sufficient in the event that the polymer is to be subjected to high temperatures and is to be used for the manufacture of clear or transparent articles, because the strong heat, even though momentary or short-lived, causes the polymer to yellow to a greater or lesser extent and this renders same unsuited for certain applications. Thus, in the manufacture of films or transparent membranes, bottles, flasks, and the like, a change in the color of the polymer which would have an adverse affect on the transparency of these final products cannot be tolerated.